1. Field of the Invention
This invention relates generally to the field of optical fiber sensors and, in particular, to the fabrication of porous sol-gel optical fibers, implantation of sensing materials for chemical and biochemical sensing in the sol-gel fibers, and the manufacture of optical fiber sensors therefrom.
2. Background of the Technology
Optical fiber chemical and biochemical sensors are based on detecting the interaction of a chemical or biochemical component with light guided by an optical fiber. See, for example, “Optical Fiber Sensor Technology: Fundamentals”, Grattan and Meggitt, eds., Kluwer Academic Publishers, Dordrecht (2000). These sensors can be used for environmental monitoring, chemical process control, and biochemical sensing. See, for example, “Optical Fiber Sensor Technology”, Volume 4: Chemical and Environmental Sensing”, Grattan and Meggitt, eds., Kluwer Academic Publishers, Dordrecht (1999) and “Chemical and Biochemical Sensing with Optical Fibers and Waveguides”, Boisd and Harmer, eds., Artech House Inc., Boston (1996). Compared with other sensors, such as electrochemical sensors, surface acoustic wave sensors, optical fiber sensors have the following advantages for chemical and biochemical sensing: small size, low cost, fast response, remote and distributed sensing capability, resistance to severe environments, and immunity to electrical noise.
According to the structure of the sensor transducer, two kinds of optical fiber sensors can be defined. In the first class of optical fiber sensor, the optical fiber itself is not a transducer. The transducer is a conventional optical cell used in spectrometry. The transducer can be a quartz cell filled with solution, a drop of aqueous solution, a small piece of porous polymer or a polymer film. In this type of sensor, an optical fiber is used to send light from a light source into the transducer and guide light from the transducer to a light detector. This kind of optical fiber sensor, however, is both insensitive and susceptible to noise from its environment. When a light beam from an optical fiber is guided to a traditional optical cell the light beam is highly diffused while traveling through the cell. Even with an efficient light collecting system the path length of the transducer still has to be limited. In addition, noise light signals from the environment are also collected and fed into the detector.
In the second class of optical fiber sensor, the cladding of the optical fiber itself can act as a transducer. A sensor of this type is disclosed by Mesica et al. in “Theory of Fiber-Optic, Evanescent-Wave Spectroscopy and Sensors”, Appl. Optics, 35, 2274 (1996). In order to manufacture such a sensor, the jacket and cladding of a conventional optical fiber are removed and a sensing material is coated on the surface of the fiber core to form a film which serves as a new cladding layer. The evanescent tail of the light guided in the fiber interacts with the sensing material in the cladding layer to yield a sensing signal. This kind of optical fiber sensor has the advantages of simple structure and immunity to environmental noise. However, the sensitivity of this kind of sensor is still limited since only a small part of the light guided through the optical fiber interacts with the sensing material. Moreover, only the sensing material within the evanescent wave field (e.g., within several micro-meters from the fiber core surface) is effective for sensing.
A significant increase in sensitivity would be expected if the fiber core rather than the cladding were used as the sensing material. Using such an active fiber core sensor, almost all of the light guided through the fiber would interact with the sensing material. In addition, since the probing light is guided within the optical fiber as it travels through the active fiber core, the problem of diffusion of the light beam is avoided and a very long active fiber core can be used as a sample cell. Also, the effect of light noise from the environment of the sensor could be avoided.
U.S. Pat. No. 5,249,251 discloses sensors having an active fiber core for fluorescence or chemiluminescence detection. U.S. Pat. No. 5,250,095 discloses a high temperature/wet chemistry procedure to make porous optical fibers. The porous optical fibers can be used as chemical sensors. The '095 patent discloses drawing a glass rod to form a fiber, treating a portion of the glass fiber to induce phase separation of the glass components and dissolving soluble components of the glass to make that portion of the fiber porous. The porous part of the fiber is then dipped in a solution to adsorb sensing material into the micro-pore in the fiber.
There still exists a need for a simple, economic and convenient method to make a porous, transparent fiber. Such a porous fiber could be used to make an optical fiber sensor by implanting a sensing material into the porous fiber.
Other objects and advantages of this invention are apparent from the specifications and drawings which follow. The application of the porous sol-gel fiber sensor is considerably broader than its use discussed herein as examples.